The interface circuit of the present invention is of the same general type as described and claimed in U.S. Pat. No. 4,039,928, which is assigned to the present assignee. However, the circuit of the invention is simpler than the circuit described in the patent and is capable of improved operation with respect thereto, as will become apparent as the description proceeds.
As described in the patent, semiconductor switching devices, such as SCR's, are presently in widespread use for controlling the current flow to high current electrical loads, such as resistance welding machines, under the control of an appropriate electronic controller which supplies control commands to the gate electrodes of the semiconductor switching devices, thereby to control the amount of current flow to the controlled equipment. Such a controller is described, for example, in copending application Ser. No. 29,752 filed Apr. 13, 1979 in the name of the present inventor, and assigned to the present assignee. A resistance welding machine, as mentioned above, is one typical example of such a high current load with which semiconductor switching devices are used to control the current fed to the machine.
The power handling capability of semiconductor switching devices of the type mentioned above is limited due to self-heating. Should the junction temperature of such a device become excessive, it can be permanently destroyed. The system described in the patent is intended to monitor the junction temperature of the semiconductor switching devices to discontinue the current through the system should the temperature reach a predetermined threshold, and to initiate an appropriate warning, thereby preventing the temperature from reaching levels which could result in the destruction of the devices.
However, the circuitry disclosed in the patent is complex, and as mentioned above, an objective of the present invention is to provide a circuit which is much simpler than the circuit of the patent, and one which achieves the same result as the circuit of the patent in an improved manner. In a more specific sense, a further objective of the invention is to provide an improved and simplified control circuit for a resistance welding machine, for monitoring the junction temperatures of the semiconductor switching devices used in controlling the operation of the machine.